Mast cells play a pivotal role in the pathogenesis of asthma and other allergic diseases. These reactions are generally initiated by antigen-dependent aggregation of the high affinity IgE receptor (Fc-epsilon-RI) expressed on the cell surface and subsequent release of pro-inflammatory mediators (e.g. histamine, prostanoids, proteases and cytokines). We have demonstrated that interferon-gamma up-regulates the high affinity receptor for IgG (Fc-gamma-RI) on cultured human mast cells and that aggregation of these receptors through IgG leads to mast cell mediator release. An object of this study is to delineate the signaling pathways of Fc-epsilon-RI and Fc-gamma-RI receptors leading to the release of inflammatory mediators from human mast cells. The current model for the signaling pathway linking Fc-epsilon-RI to mast cell activation has largely been based on studies conducted in RBL 2H3 cells. RBL 2H3 cells are a transformed rodent cell line, which may not be, in all aspects, reflective of the mature human mast cells. The signaling pathways linking the Fc-gamma-RI receptor to mast cell activation have yet to be delineated. It is now possible to grow human mast cells from a CD34-positive, pluripotent cell population in primary culture with sufficient number and purity to study defined signaling events. Thus, we are now examining specific signaling pathways involved in Fc-epsilon-RI- and Fc-gamma-RI- dependent inflammatory mediator release from human mast cells.